1. Field of the Invention
The invention concerns a rectifier switch with series-connected thyristors, which exhibit a considerable spread of at least 10% in their forward and reverse blocking voltages, especially for high-voltage, high-power rectifiers.
2. Description of the Prior Art
The forward and reverse blocking voltages of the thyristors of one or more production batches, made by the production techniques customary today, exhibit a large spread of about 30%-40%. FIG. 1 shows diagrammatically the spread of the forward and reverse blocking voltages of the thyristors of a lot. ST is the number of thyristors classified as to voltage and laid out along the ordinate axis, the abscissa indicates the voltage and the forward blocking voltage U.sub.BI is in the positive direction while the breakdown voltage U.sub.D is in the negative direction. The vertical lines S.sub.pN bound the voltage classes e.g. for low-voltage applications. FIG. 1a shows the current-voltage characteristic of a thyristor.
In view of the great voltage spread of thyristors, it is customary in low-voltage technology to use only thyristors of one voltage class for a rectifier, as FIG. 1 shows. In high-voltage-direct-current transmission systems, among others, thyristors of high breakdown strength are used to an increasing extent. The classification of such thyristors at present establishes the rated voltage of the thyristors of the production run according to the thyristor with the lowest breakdown strength; thus a large proportion of the thyristors is poorly utilized in terms of voltage. FIG. 2 shows the relevant conditions for the spread in the forward and reverse blocking voltages of the thyristors of lot, e.g. for high-voltage-direct-current transmission (HVDC). NSPG is the rated voltage of the thyristors.
Based on the above statements, there is a need to use as many as possible of the serviceable thyristors of a batch for the high-power switches of an installation and to utilize all thyristors fully, voltagewise. This has its problems, however, because with full voltage loading of the thyristor chain there occurs an unequal voltage distribution across the individual thyristors - due to their individual and their dynamic characteristics as well as to external influences (ground capacities, component tolerances).